TW201625493A - Method for producing a blank of fluorine- and titanium-doped glass with a high silicic-acid content for use in EUV lithography and blank produced according to said method - Google Patents

Abstract

The present invention refers to a method for producing a silica glass blank co-doped with titanium and fluorine for use in EUV lithography, with an internal transmission of at least 60% in the wavelength range of 400 nm to 700 nm at a sample thickness of 10 mm, the titanium being present in the oxidation forms Ti3<SP>+</SP> and Ti<SP>4+</SP>, the method comprising the following steps: (a) producing a TiO2-SiO2 soot body by means of flame hydrolysis of silicon- and titanium-containing precursor substances, (b) fluorinating the soot body so as to form a fluorine-doped TiO2-SiO2 soot body, (c) treating the fluorine-doped TiO2-SiO2 soot body in a water vapor-containing atmosphere so as to form a conditioned soot body, and (d) vitrifying the conditioned soot body so as to form the blank of titanium-doped silica glass with a mean OH content in the range of 10 to 100 wt. ppm and with a mean fluorine content in the range of 2,500 to 10,000 wt. ppm. Also after the action of reducing atmosphere on the blank the OH groups introduced by the water vapor treatment show a prolonged action period in the sense of a depot for the internal oxidation of Ti<SP>3+</SP> ions into Ti<SP>4+</SP> ions, so that the ratio of Ti<SP>3+</SP>/Ti<SP>4+</SP> can be adjusted to a value ≤ 2 x 10<SP>-4</SP>.

Description

Method for producing a fluorine and titanium doped glass substrate having a high tannic acid content for EUV lithography and a substrate manufactured by the method

The present invention relates to a method for fabricating a titanium-doped glass substrate for EUV lithography having a high tannic acid content and having an internal transmission of at least 60 in the wavelength range of 400 nm to 700 nm at a sample thickness of 10 mm. % with a given fluorine content.

Furthermore, the present invention relates to a bottom plate of a titanium-doped vermiculite glass for EUV lithography.

In EUV lithography, a highly integrated structure with a line width of less than 50 nm is fabricated using a microlithographic projection device. Radiation from the EUV range (extremely ultraviolet, also known as weak X-ray radiation) having a wavelength of usually 13 nm is used here. The projection apparatus is equipped with a titanium doped glass having a high tannic acid content (hereinafter also referred to as "TiO ₂ -SiO ₂ glass" or "Ti doped vermiculite glass") and is provided with a reflective layer system. Mirror element. These materials are known for their extremely low linear thermal expansion coefficient (referred to as "CTE"; thermal expansion coefficient), and the thermal expansion coefficient can be adjusted by the concentration of titanium. The standard titanium oxide concentration is between 6% and 9% by weight.

In the intended use of a base plate made of a synthetic titanium doped glass having a high tannic acid content, the upper side is provided with a reflective coating. The maximum (theoretical) reflectance of such EUV mirror elements is about 70%, so at least 30% of the radiant energy is drawn into the coating or into the near surface layer of the mirror substrate and converted to heat. In the volume of the mirror substrate, this results in an uneven temperature distribution with a temperature difference, which can be as much as 50 ° C according to the information given in the literature.

Therefore, if the CTE of the glass of the substrate of the mirror substrate is zero during the temperature range of the entire working temperature during use, it is desirable that the deformation be as small as possible. However, in Ti-doped vermiculite glass, the temperature range in which CTE is about zero is substantially very narrow.

The temperature at which the coefficient of thermal expansion of the glass is equal to zero will also be referred to hereinafter as the zero crossing temperature or T _ZC (temperature across zero). The titanium concentration is typically set such that a CTE of zero is obtained over a temperature range between 20 °C and 45 °C. The volume region of the mirror substrate having a higher or lower temperature than the predetermined T _ZC expands or contracts, and thus, despite the overall low CTE of the TiO ₂ -SiO ₂ glass, deformation which is detrimental to the image quality of the mirror may occur.

In addition, the virtual temperature of the glass is crucial. The virtual temperature is a glass property that represents the state of the order of "freezing" the glass network structure. The higher the temperature of TiO ₂ -SiO ₂ glass virtual accompanying low state of order of the glass structure and a greater deviation of the energy of the most favorable arrangement structure.

The virtual temperature is affected by the thermal experience of the glass, especially by the final cooling process. In the final cooling procedure, the near surface area of the glass block must be prioritized over the central area under different conditions, so the different volume areas of the mirror substrate bottom plate have different virtual temperatures due to their different thermal experiences, and the virtual temperature is followed by the CTE curve. Corresponding to the uneven area correlation. Furthermore, however, since the fluorine has an effect on structural relaxation, the virtual temperature is also affected by the amount of fluorine. Fluorine doping allows adjustment of the low virtual temperature and therefore also allows the CTE curve to have a small slope with respect to temperature.

Ti-doped vermiculite glass is produced by flame hydrolysis starting from a precursor containing barium and titanium. First, a titanium-doped SiO ₂ porous soot body is produced and vitrified into a dense glass body. Optionally, the soot body is subjected to a drying procedure prior to vitrification, for example by treatment in a halogen-containing atmosphere to reduce the hydroxyl content (OH group content). However, doping with titanium oxide results in a brown appearance or staining of the glass due to the substantially high concentration of Ti ³⁺ ions in the glass matrix. The shaped bodies for this application (hereinafter also referred to as bottom plates) are large dark brown plates up to a size of up to about 70 x 60 x 20 cm ³ , which must be tailored to the optical properties of the plates and to the defects or defects caused by the manufacturing process. Check evenly. The general optical measurement method that presupposes transparency in the visible spectral range can only be used with limited or no application at all, and the brown appearance of the glass becomes a problem.

This document has proposed various solutions for the benefit of Ti ⁴⁺ ions by oxidizing treatment for limiting the amount of Ti ³⁺ ions. When Ti-doped vermiculite glass is used at a relatively high hydroxyl content, the OH groups can oxidize Ti ³⁺ to Ti ⁴⁺ as desired. This is, for example, a description of Ti-doped vermiculite glass by Carson and Mauer in "Optical Attenuation in Titania-Silica Glasses" (J. Non-Crystalline Solids, Vol. 11 (1973), pp. 368-380). It indicates the reaction according to the formula 2Ti ³⁺ +2OH ^- → 2Ti ⁴⁺ + 2O ^2- + H ₂ .

This process is employed in EP 2 428 488 A1, in particular with regard to the oxidation procedure and the optimum conditions for the outward diffusion of hydrogen during annealing. The Ti-doped vermiculite glass described in EP 2 428 488 A1 is not fluorine doped, has a high OH content of more than 600 wt. ppm and a relatively low hydrogen content (less than 2 x 10 ¹⁷ molecules/cm ³ ). In order to ensure a high OH content, it is recommended to add water vapor during the deposition procedure, thus describing a two-stage deposition procedure in which TiO ₂ -SiO ₂ soot particles are formed first, which is equivalent to subsequent consolidation and vitrification, and describes a one-stage procedure in which soot The particle system is immediately vitrified (so-called "direct quartz" or "DQ method"). The amount of Ti ³⁺ ions in the Ti-doped vermiculite glass is said to be less than 3 ppm, and the internal transmission in the wavelength range of 340 nm to 840 nm is greater than 90%, but no information on the thickness of the sample is provided.

WO 2004/089836 A1 discloses a fluorine-doped Ti-doped vermiculite glass which exhibits a very flat thermal expansion coefficient slope over a relatively wide temperature range. First, a porous TiO ₂ -SiO ₂ soot body was prepared at 1200 ° C in air, which involved a decrease in OH content and oxidation of Ti ³⁺ ions. Subsequently, to fluorine-doped, the TiO ₂ -SiO ₂ soot on the exposure system having an oxygen or 10% by volume of SiF ₄ hours in an atmosphere of helium. In addition to fluorine doping, this treatment involves a further reduction in OH content. In order to prevent darkening or dyeing of the soot body during vitrification, it is proposed in WO 2004/089836 A1 that the soot body should be at 300 ° C and 1300 in an oxygen atmosphere prior to vitrification prior to subsequent vitrification. It is treated for several hours at a temperature range of °C. The glass body of fluorine and titanium doped vermiculite glass is then formed into a bottom plate and annealed to set a virtual temperature. No information on the amount of Ti ³⁺ ions or on dark dyeing or on internal transmission can be found in WO 2004/089836 A1.

WO 2006/004169 A1 continues with an example of the amount of Ti ³⁺ and information about internal transmission in WO 2004/089836 A1. The method according to WO 2006/004169 A1 also provides an oxygen treatment of the TiO ₂ -SiO ₂ soot body with fluorine doping prior to vitrification (under the underarm). The fluorine doping is carried out in an atmosphere containing oxygen and fluorine. The Ti-doped vermiculite glass produced in this manner contains 10 wt. ppm of OH groups and 12 wt. ppm of Ti ³⁺ ions. The fluorine content was 120 wt. ppm and 6,300 wt. ppm, respectively. In the case of this relatively high content of Ti ³⁺ , the internal transmission in the wavelength range from 400 nm to 700 nm is specified to be more than 80%, however this is for a glass thickness of only one millimeter. Converted to a sample having a thickness of 10 mm, this corresponds to a value of transmission within only 10%.

The method according to WO 2004/089836 A1 and WO 2006/004169 A1 is technically very complicated and, in terms of the actual sample thickness in the range of 10 mm, does not produce an sufficiently high internal transmission acceptable.

It is known from US 2006/0179879 A1 that in TiO ₂ -SiO ₂ glass for EUV lithography, the CTE curve obtained with temperature during operation is subject to other parameters in addition to the uniform distribution of titanium concentration. The effect is especially affected by fluorine doping and by OH content. Fluorine can also be used as a dry reagent which can be used to set the OH content to less than 100 ppm. Conversely, an OH content of up to 1500 ppm is obtained via the action of water vapor during vitrification. In a particular embodiment according to US 2006/0179879 A1, a fluorine-doped TiO ₂ -SiO ₂ soot system is obtained by flame hydrolysis of a precursor material comprising cerium, titanium and fluorine. In a subsequent process step, the soot system is vitrified or consolidated in an inert atmosphere containing water vapor. The fluorine content of this TiO ₂ -SiO ₂ glass is in the range of 500 wt. ppm to 2000 wt. ppm. Information on the amount of Ti ³⁺ ions in the TiO ₂ -SiO ₂ glass, on the OH content, and the internal transmission of the fluorine-doped TiO ₂ -SiO ₂ glass in the visible wavelength range is not provided.

In addition to the specific examples described above, US 2006/0179879 A1 also discusses the manufacture of quartz glass according to the so-called soot method, under the heading "Soot Formation Followed by Consolidation". Thus, in the case where fluorine doping is required, the SiO ₂ soot body can be subjected to helium, hydrogen, water vapor or a doping gas (such as CF) at about 1000 ° C before the subsequent vitrification step at a higher temperature. ₄ ) Processing. The effect on treatment with hydrazine, hydrogen or steam for SiO ₂ soot bodies or for sintered quartz glass is not provided. However, it must be assumed that since the drying of the soot body prior to vitrification is clearly not intentional, a high OH content is present in the soot body, which is increased by the aqueous vapor atmosphere and will result in unwanted bubbles during vitrification. .

The mentioned WO 2009/084171 A1, US 2010/0179047 A1, US 2014/0155246 A1 and EP 2 377 826 A1 as fluorine co-doped with respect to the TiO ₂ -SiO ₂ Further prior art publications glass.

Technical purpose

In summary, it should be noted that according to the prior art, the Ti ³⁺ ion in the Ti-doped vermiculite glass is reduced to facilitate the Ti ⁴⁺ ion to be ensured by the internal oxidation of hydrogen diffusion by a sufficiently large amount of OH groups, or At low OH group levels, oxidation treatment is required prior to vitrification, which requires high processing temperatures as well as special corrosion resistant furnaces and is therefore relatively expensive.

In the F-doped TiO ₂ -SiO ₂ glass, the problem of brown coloring or dyeing caused by high Ti ³⁺ ions is particularly remarkable because it is substantially free of OH groups due to fluorine and can induce Ti ^{3 again. +} Oxidation to Ti ⁴⁺ .

Furthermore, it has been found that although the known oxygen treatment prior to vitrification reliably increases the amount of oxygen, it occurs once to facilitate oxidation of the Ti ⁴⁺ ions, for example by being vitrified in a reducing atmosphere (for example by It is not permanent when it is formed by applying a hydrogen-oxygen flame adjusted in a reducing manner. This means that due to the oxidation treatment, oxygen can only be used to oxidize Ti ³⁺ to Ti ⁴⁺ once, so under reducing conditions, it gradually increases to form Ti ³⁺ ions, which is known to cause dark appearance or dyeing of the glass. .

Accordingly, the present invention is directed to an inexpensive manufacturing method for glass doped with titanium and fluorine and having a high tannic acid content and an OH content of less than 100 wt. ppm, showing an internal transmission of a sample thickness of 10 mm in a wavelength range of 400 nm to 700 nm. At least 60%, wherein after the reducing atmosphere is applied to the glass, the OH group is shown to be prolonged during use as a ^reservoir for Ti ³⁺ internal oxidized Ti ⁴⁺ ions.

Further, it is an object of the present invention to provide a vermiculite glass substrate co-doped with titanium and fluorine.

General description of the invention

With regard to this method, the object starting from the above-described method is obtained according to the invention having the following method steps: (a) TiO ₂ -SiO ₂ soot body is produced by flame hydrolysis of a ruthenium-containing and titanium-containing precursor substance, (b) The ash body is fluorinated to form a fluorine-doped TiO ₂ -SiO ₂ soot body, (c) the fluorine-doped TiO ₂ -SiO ₂ soot body is treated in an aqueous vapor atmosphere to form a conditioned soot body (d) vitrifying the conditioned soot body to form a high bismuth acid content, titanium having an average hydroxyl content in the range of 10 to 100 wt. ppm and an average fluorine content ranging from 2,500 wt. ppm to 10,000 wt. ppm. The bottom plate of the miscellaneous glass.

In the production of a synthetic Ti-doped vermiculite glass according to a so-called "soot method" by flame hydrolysis, SiO ₂ and TiO ₂ produced by hydrolysis or oxidation in the flame are formed when a TiO ₂ -SiO ₂ soot body is formed. The particles are first deposited on the deposition surface (method step (a)). As an alternative to the "soot method" according to the present invention, the Ti-doped vermiculite glass can also be directly vitrified according to the SiO ₂ and TiO ₂ particles deposited therein, and generally obtained in the range of about 450 to 1200 wt. ppm. The first step of the OH content is "direct method". However, the Ti-doped vermiculite glass produced by this direct method is not the gist of the present invention.

In a further method step (b), the soot system is doped with fluorine, so that the fluorine content in the range of 2500 wt. ppm to 10,000 wt. ppm is set in the vitrified bottom plate. The hydroxyl group is substantially removed by fluorination. This state has the disadvantage that there are no more hydroxyl groups for the oxidation of Ti ³⁺ to Ti ⁴⁺ and that the bottom plate made of fluorine-doped TiO ₂ -SiO ₂ glass is expected to have strong brown coloration or dyeing.

In order to prevent or at least reduce brown dyeing, the fluorination of the TiO ₂ -SiO ₂ soot body is carried out in method step (c) followed by conditioning in an aqueous vapor atmosphere, whereby sufficient oxidation of Ti ³⁺ to Ti is obtained. The amount of hydroxyl groups of ^{4+ is adjusted to} the soot body.

The conditioned soot body is then vitrified in forming a titanium doped glass substrate having a high tannic acid content and an OH content in the range of 10 wt. ppm to 100 wt. ppm (method step (d)).

The central idea of the method according to the invention is to facilitate Ti ⁴⁺ by reducing the concentration of Ti ³⁺ ions by oxidation conditioning treatment using water vapor prior to vitrification of the TiO ₂ -SiO ₂ soot body. The conditioning treatment using water vapor is carried out prior to the vitrification step, since open-cell soot bodies in which the hydroxyl groups can be easily incorporated remain at this stage. The incorporation of hydroxyl groups is carried out here so that the internal oxidation of Ti ³⁺ to Ti ⁴⁺ has the effect of storage for subsequent process steps under reducing conditions. A particularly uniform distribution of hydroxyl groups in the soot body is also obtained in this conditioning treatment. The fluorine coordination in the glass particles is substantially maintained here, so that the TiO ₂ -SiO ₂ substrate which is doped with fluorine and at the same time has a sufficiently high OH content is produced according to the method of the invention to ensure the oxidation of Ti ³⁺ to Ti ⁴⁺ .

Due to the conditioning treatment, the OH group is incorporated into the soot body as a storage, so they are available for internal oxidation more than once, and are also effective after applying reducing conditions in subsequent method steps, at relatively short temperatures lasting several days. Re-indoor oxidation during processing, depending on the volume of the substrate, is in the range between 600 ° C and 1000 ° C, usually in air or under vacuum, which in turn minimizes the amount of Ti ³⁺ ions .

The Ti-doped vermiculite glass to be produced according to the process of the invention contains titanium dioxide in the range of from 6% by weight to 12% by weight, which corresponds to a titanium content of from 3.6% by weight to 7.2% by weight. In the TiO ₂ -SiO ₂ soot body, at least partially oxidized form in the presence of titanium Ti ^3+. If possible, it is desirable to convert all Ti ³⁺ ions into Ti ⁴⁺ ions, so there is no unsuitable absorption in the wavelength range of 400 nm to 1000 nm due to Ti ³⁺ ions, and the Ti doped vermiculite glass This shows the maximum transparency in this wavelength range. Since the soot body does not have an OH group or only a small amount (<10 wt. ppm) of OH groups due to fluorination doping, this has almost no benefit for the oxidation of Ti ³⁺ to Ti ⁴⁺ . As the oxidizing treatment agent, conditioning treatment using an atmosphere containing water vapor is carried out according to the present invention before the vitrification step. The open-cell soot body also reacts with water vapor at relatively low temperatures to convert Ti ³⁺ ions into Ti ⁴⁺ ions. The OH group is incorporated as a Si-OH into the glass network structure. The OH groups are still available for internal oxidation of Ti ³⁺ to Ti ⁴⁺ after subsequent treatment in a reducing atmosphere (such as in a graphite furnace).

When water vapor is used only during vitrification as in the prior art, the effect is rather small, and the reaction between the OH group and the Ti ³⁺ ion is irregular, causing the inner and outer surfaces to gradually shrink, thereby allowing water from the water. The OH group penetration of the vapor and the reaction are hindered. Quite unexpectedly, water vapor causes bubbles to form in the glass, which is unacceptable for a method of making a substrate from titanium doped glass having a high tannic acid content.

In an oxygen atmosphere compared to the TiO ₂ -SiO ₂ soot-processing, from conventional prior art, the use of steam conditioning process technology is not very complicated, and very useful in the process according to the present invention. With the method of the present invention, the conditioning treatment can be carried out in a vacuum or/or in an inert gas in a glass or ceramic furnace which is also used for drying and vitrifying the soot body. The method according to the invention is thus particularly economical. Furthermore, it is advantageous to set the OH content between 10 wt. ppm and 100 wt. ppm, which results in a particularly high uniformity of CTE and virtual temperature. When the OH content exceeds 100 ppm, the distribution of the OH group is thus irregular and even at a particularly high OH content, bubble formation in the glass can be expected.

Furthermore, the method of the invention using water vapor should be preferred over the conditioning treatment using nitrogen oxides which produce the desired Ti ³⁺ oxidation to Ti ⁴⁺ as disclosed in DE 10 2013 108 885 B2. Some nitrogen oxides are toxic and quite harmful to the environment, and in addition to efficiency, they require a higher temperature than water vapor. Water vapor has the particular advantage that it can be obtained in high purity on a large industrial scale and is harmless.

When water vapor decomposes, a reactive OH group that has reacted with Ti ³⁺ ions at a relatively low temperature is formed. The reaction of Ti ³⁺ ions with water vapor is carried out according to the following reaction formula (1) and releases water vapor (H ₂ ): (1) 2 Ti ³⁺ + H ₂ O → 2 Ti ⁴⁺ + O ^2- + H ₂

Introducing the OH group of the fluorine and titanium doped vermiculite glass by steam conditioning treatment causes the Ti ³⁺ ion to internally oxidize the Ti ⁴⁺ ion more than once, but if the reducing atmosphere acts on the vitrified bottom plate at a high temperature Ti ³⁺ ions which are deteriorated in transmission in the bottom plate are also available for re-internal oxidation (storage effect) when formed in this step again.

Thus, the results can be used in this state, when the base plate processing line after the vitrification according to method step (d) in a reducing atmosphere of, and Ti ³⁺ / Ti ⁴⁺ with the ratio based on the wavelength range of 400nm to 700nm The internal transmission is reduced and increased, and then the substrate is annealed at a temperature ranging from 600 ° C to 1000 ° C to reverse the decrease in internal transmission. The effect of the reducing atmosphere is observed, for example, during the formation of the bottom plate in the graphite mold, and the brown coloration of the bottom plate is caused by the reduction of Ti ⁴⁺ ions into Ti ³⁺ ions. Brown dyeing can be largely eliminated again by annealing, for example in air or in a vacuum, at an annealing temperature between 600 ° C and 1000 ° C, because the OH groups incorporated into the glass can be again for internal oxidation of Ti ^3+, so that the Ti ³⁺ / Ti ⁴⁺ ratio set 2 × 10 ^-4 . This annealing treatment does not involve the action of gas on the bottom plate, but involves outward diffusion of hydrogen as a reaction product of re-internal oxidation according to formula (1).

Furthermore, it can in turn be used when the conditioning treatment using steam is carried out at a temperature in the range of from 100 ° C to 1100 ° C, preferably in the range of from 500 ° C to 1000 ° C for a period of one (1) to 40 hours. Time.

Since the conditioning treatment has been carried out at a temperature of 100 ° C, the oxidation of Ti ³⁺ ions may consume a relatively small amount of energy. Therefore, this only requires the use of a furnace of a rather simple design, and this processing stage can be easily repeated. At temperatures below about 1100 ° C, the porous structure of the soot body remains, thus ensuring that the gaseous treatment agent can diffuse through the soot body and react uniformly with the Ti ³⁺ ions distributed in the glass network structure. At relatively low temperatures ranging from 100 ° C to 500 ° C, the infiltration of water vapor into the soot body requires a relatively long treatment period until the desired oxidation reaction with Ti ³⁺ ions occurs and the hydroxyl groups from the water vapor Accumulated on the surface of individual soot particles. Depending on the processing temperature, the processing period also depends on the volume of the soot body. As a result, it has been found that a minimum treatment period of at least one hour is suitable for ensuring efficient infiltration of water vapor into the soot body. Thereby, the treatment gas is substantially uniformly distributed in the porous soot body. Water vapor can be introduced into the soot body using an inert carrier gas stream.

Advantageously, the amount of water vapor in the inert gas during the conditioning treatment is from 0.05 to 50% by volume, preferably from 1 to 20% by volume.

If the water vapor fraction is less than 0.05% by volume, the oxidation effect is low, and if the water vapor fraction is higher than 50% by volume, surface water is formed on the soot body, which is harmless in principle, but must be moved again before vitrification. except.

The results showed that after fluorination and prior to producing TiO ₂ -SiO ₂ soot body according to method step (b) is useful for drying, to produce an OH content lower than the average of 10wt.ppm. By this dehydration treatment, the water accumulated in the soot body is removed, which allows a particularly uniform distribution of fluorine in the subsequent fluorination step. Drying can be carried out in a purely hot manner in an inert gas, dry air or under vacuum at temperatures ranging from 700 ° C to 1100 ° C; alternatively, the use of dry reagents such as chlorine is also standard practice. This drying step facilitates reducing the OH content to less than 10 wt. ppm. After this drying step, the system TiO ₂ -SiO ₂ soot doped with fluorine, resulting in a further drying effect. Therefore, the soot body has an OH content of less than 1 wt. ppm, which is in a state involving a high Ti ^{3 +} ion content of 20 ppm to 30 ppm. If the soot body is vitrified without further treatment, the bottom plate shows intense dark dyeing.

Fluorine has an effect on the structural relaxation of vermiculite glass, so the virtual temperature of the state of the order of the Ti-doped vermiculite glass as a "frozen" glass network structure can be reduced, and the temperature range can be expanded by zero thermal expansion. The coefficient is expanded. This is known, for example, from Journal of Applied Physics (Vol. 91 (8), April 2002, pages 4886-4890). Depending on the processing and temperature, a concentration of fluorine between 2,500 wt. ppm and 10,000 wt. ppm, preferably between 5,000 wt. ppm and 10,000 wt. ppm, is set in the bottom plate.

Another advantageous configuration of the invention consists in carrying out a fluorine treatment of the TiO ₂ -SiO ₂ soot body according to process step (b) in a fluorine-containing atmosphere, which preferably contains from 2% by volume to 100% by volume of SiF ₄ . In principle, the use of pure fluorine gas (F ₂₎ or SiHF ₃ or SiH ₂ F ₂ instead of SiF _4.

The use of such fluorinated gases due to the reduction of carbon-containing fluorinated gases such as CHF ₃ , CF ₄ , C ₂ F ₆ or C ₃ F ₈ , which would support the formation of unwanted Ti ³⁺ ions Quite disadvantageous.

Furthermore, it is advantageous when the fluorination according to process step (b) is carried out in a temperature range of from 700 ° C to not more than 1000 ° C. By fluorination in this temperature range, the porous soot body is easily permeable to the fluorine-containing process gas, thereby ensuring efficient incorporation of fluorine into the glass network structure.

The fluorine-doped TiO ₂ -SiO ₂ substrate advantageously has an average fluorine content in the range of 2,500 to 10,000 wt. ppm with respect to a particularly flat zero coefficient of thermal expansion coefficient in the temperature range of 20 ° C to 50 ° C.

With regard to a substrate composed of Ti-doped glass having a high tannic acid content, the above object is attained from a substrate of the above type, wherein the substrate is produced according to the following method steps: (a) using a ruthenium-containing and titanium-containing precursor material The flame is hydrolyzed to produce a TiO ₂ -SiO ₂ soot body, (b) the soot body is fluorinated to form a fluorine-doped TiO ₂ -SiO ₂ soot body, and (c) the fluorine doped body is treated in an aqueous vapor atmosphere. a TiO ₂ -SiO ₂ soot body to form a conditioned soot body, and (d) a glass substrate of the conditioned soot body to form a high tannic acid content titanium doped glass, which is known by the following: The fluorine content is in the range of 2,500 wt. ppm to 10,000 wt. ppm, the average OH content is in the range of 10 to 100 wt. ppm, and the average TiO ₂ content is in the range of 6 to 12 wt%, wherein the titanium is Ti ³⁺ And the oxidized form of Ti ⁴⁺ exists, and the ratio of Ti ³⁺ /Ti ⁴⁺ is adjusted to 2 × 10 ^-4 value.

Due to the small amount of Ti ³⁺ ions, the substrate according to the present invention exhibits a high transparency of more than 60% in the wavelength range of 400 nm to 700 nm in terms of a sample having a thickness of 10 mm. It is thus possible to inspect the bottom plate without difficulty using standard optical measurement methods.

The concentration of Ti ³⁺ can be measured by electron spin resonance (as published by Carson and Mauer in "Optical Attenuation In Titania-Silica Glasses" (J. Non-Crystalline Solids, Vol. 11 (1973), pp. 368-380). ) Determination.

In addition, the bottom plate of the Ti-doped vermiculite glass is doped with fluorine. The fluorine content is in the range of 5,000 wt. ppm to 10,000 wt. ppm in terms of a particularly flat zero coefficient of thermal expansion coefficient in the temperature range of 20 ° C to 50 ° C.

The average fluorine concentration is usually determined in a wet chemical method. Will be based on this The measured sample of the bottom plate of the invention was first dissolved in an aqueous NaOH solution. The F concentration is obtained by measuring the electromotive force of the dissolved measurement sample using an electrode sensitive to fluorine.

The average hydroxyl content (OH content) is measured in accordance with the IR absorption according to the method of D. M. Dodd et al. ("Optical Determinations of OH in Fused Silica", (1966), p. 3911).

Furthermore, the substrate produced according to the method of the invention shows a very advantageous progression of the coefficient of thermal expansion CTE with a small slope in the temperature range from 20 °C to 40 °C. The CTE slope expressed as the derivative quotient dCTE/dt is less than 1.0 ppb/K ² . Furthermore, such a substrate made from fluorine and titanium doped vermiculite according to the method of the invention is distinguished by a particularly high dopant distribution uniformity. This optimizes the CTE local curve in the optical use zone (also known as the "CA zone" (pass light aperture)).

In the substrate fabricated in accordance with the present invention, a relatively low virtual temperature contributed by fluorine doping is additionally detected.

The base plate made in accordance with the present invention is most suitable for EUV lithography applications. Also due to its transparency in the visible spectral range, an optimal inspection may be performed prior to further processing steps, for example to obtain a mirror substrate. Due to the storage effect of the OH group incorporated by steam treatment, the transparency in the visible spectrum is substantially maintained after the treatment step in the reducing atmosphere is repeated, or compared to the initial value, by air or It can be rebuilt or even exceeded after a particularly enhanced reduction after annealing in a range between 600 ° C and 1000 ° C under vacuum.

Implementation aspect

The invention is explained in more detail with reference to the embodiments and drawings. In the drawings, each time is compared to a material that has not been subjected to the conditioning treatment of water vapor of the present invention.

Figure 1 is a graph showing the internal transmission before and after the forming step of the substrate produced by the method of the present invention under a reducing atmosphere; Figure 2 shows a CTE plot relative to temperature (10 ° C to 70 ° C).

Example 1

The soot system is produced by flame hydrolysis of octamethylcyclotetraoxane (OMCTS) and titanium isopropoxide [Ti(OPr ⁱ ) ₄ ] by means of the known "external vapor deposition method (OVD method)". The soot system consists of a synthetic vermiculite glass doped with 8 wt% of TiO ₂ .

Now in an atmosphere containing 50% by volume of SiF ₄ in this TiO ₂ -SiO ₂ soot body doping treatment composition and dried.

The treatment was carried out at a temperature of 900 ° C for a period of 10 hours. This results in the fluorine being firmly incorporated into the TiO ₂ -SiO ₂ soot body to be vitrified. The 3-hour treatment period was applied to the conditioning treatment of a water vapor atmosphere having 2% by volume of H ₂ O at a temperature of 800 ° C.

The steam-treated fluorinated TiO ₂ -SiO ₂ soot body is then subsequently grown in a sintering furnace at a temperature of about 1400 ° C in a crucible or under vacuum (about 10 ^-2 mbar) for 5 hours. A transparent Ti-doped vermiculite glass bottom plate in the form of a rod. This bottom plate shows a very small amount of Ti ³⁺ ions of only about 6 wt. ppm and is known for an average fluorine content of 6,000 wt. ppm and an OH content of 60 wt. ppm. The first measurement of the 10 mm sample thickness was measured for internal transmission in the wavelength range of 400 nm to 700 nm (see Figure 1, curve 1.0), which is in the range of 60% to 70%.

The vitrified bottom plate is then homogenized by thermomechanical homogenization by twisting under the action of a reducing oxyhydrogen flame. The rod-shaped bottom plate assumes a barrel shape and exhibits a slightly increased brown coloration, which is accompanied by a transmission value of about 50% in the visible spectral range (sample thickness 10 mm).

This is followed by a procedure for further forming a cylindrical body. The bottom plate was placed into a melting mold of graphite having a bottom of a circular or polygonal cross section and having an outer diameter of about 300 mm. In order to carry out the forming process, the entire melting mold in which the bottom plate is positioned is first heated to 1250 ° C, then heated to 1600 ° C at a heating rate of 9 ° C / min, and then heated to 1680 ° C at a heating rate of 2 ° C / min. temperature. The vermiculite glass block is maintained at this temperature until the softened Ti-doped vermiculite glass flows out of the bottom of the melting mold under its own weight to fill the mold. A circular or polygonal plate having a thickness of about 60 mm was formed from the bottom plate, and the plate was free from delamination or grain lines as seen from the three viewing directions. After the forming step in the reducing atmosphere, a ratio of Ti ³⁺ or Ti ³⁺ /Ti ⁴⁺ of 9 wt. ppm was detected in the bottom plate to be about 2.5 × 10 ^-4 . The transmission in the visible spectral range on the substrate sample having a thickness of 10 mm is in the range of about 40% and 50%.

In order to reduce mechanical strain and avoid birefringence, the Ti-doped vermiculite glass substrate is annealed, wherein the cylindrical bottom plate is heated in air and heated to 950 ° C under atmospheric pressure for a period of 8 hours, followed by 1 The cooling rate of ° C/h was cooled to a temperature of 700 ° C and maintained at this temperature for 4 hours. Subsequently, cooling to 300 ° C was carried out at an increased cooling rate of 50 ° C / h, at which time the furnace was turned off and the bottom plate was freely cooled in the furnace. After this annealing temperature, an average virtual temperature ( _Tf ) of 800 °C was obtained.

The standard measurement method for measuring the virtual temperature by means of Raman scattering intensity at a wavenumber of about 606 cm ^-1 is described in Ch. Pfleiderer et al. "The UV-induced 210 nm absorption band in fused silica with different thermal history and Stoichiometry" (Journal of Non-Cryst. Solids 159 (1993), pp. 143-145).

Due to the annealing treatment in the air, the effect of the OH-based reservoir incorporated by the steam conditioning treatment is activated, so that the substrate even appears brighter than after the initial vitrification. According to Figure 1, curve 1.1, the internal transmission of the substrate produced according to the method of the invention has an average internal transmission of 80%.

Internal transmission represents the transmission of the entire sample thickness by the amount of corrected surface loss.

Furthermore, with respect to the base plate produced according to the method of the invention, the average coefficient of thermal expansion is by means of R. Schödel's "Ultra-high accuracy thermal expansion measurements with PTB's precision interferometer" (Meas. Sci. Technol. 19 (2008) 084003 ( The method described in 11 pp)) is determined by an interferometry method.

In the substrate manufactured in accordance with the present invention, a cross-zero temperature (T _ZC ) of 28 ° C and a CTE slope of 0.8 ppb / K ^{2 were} measured.

Since the bottom plate exhibits relatively strong stress birefringence in its edge portion, a portion having a larger dimension than the outline of the assembly, in other words, a thickness of 3 mm, is removed from the front. The bottom plate is known for its high transparency in the visible spectral range due to the ratio of Ti ³⁺ /Ti ⁺⁴ of 0.7 × 10 ^-4 , and can now be examined by standard optical measurement methods and further processed according to the obtained measurement results.

The graph of Figure 2 shows the coefficient of thermal expansion CTE as a function of temperature. Producing a fluorine-doped according to the method of the present invention the TiO ₂ -SiO ₂ base especially a CTE of a flat distribution is evident from curve 1. The gradient of CTE at a cross-zero temperature of 28 ° C was 0.8 ppb / K ² .

Comparative example 1

Under conditions as in Example 1 explained in more detail, for producing TiO ₂ -SiO ₂ soot body, and an atmosphere containing 20 vol% of SiF ₄ in combination drying and doping treatment.

This treatment was carried out at a temperature of 900 ° C for 10 hours and caused the fluorine to be firmly incorporated into the TiO ₂ -SiO ₂ soot body to be vitrified. The oxygen treatment was then carried out on a soot body at 1000 ° C for 4 hours under atmospheric pressure in a 100% oxygen atmosphere. The vitrification was then carried out at 1550 ° C in a helium atmosphere. The obtained substrate thus has an OH content of less than 1 wt. ppm (below the detection limit), but is not yet in the visible spectrum due to the relatively favorable ratio of Ti ³⁺ to Ti ⁴⁺ of 2 × 10 ^-4 A good transmission between about 60% and 75% is shown, as shown by curve 2.0 in Figure 1. The substrate must now be homogenized and formed. These subsequent process steps are carried out under a reducing atmosphere. After homogenization, intense brown staining of the bottom plate can be detected, which is exacerbated in subsequent forming procedures. Annealing at 1000 ° C in air does not alter this brown staining. Due to the homogenization and the main reducing atmosphere during the formation, the Ti ³⁺ /Ti ⁴⁺ ratio is shifted to favor Ti ³⁺ ions and is about 2.5×10 ^-4 . Thus, a sample block having a bottom plate having a thickness of 10 mm shows an irreversible reduction in internal transmission to a value of about 45%, as shown in curve 2.1 of FIG. This property cannot be improved by annealing in air, since virtually no oxygen can penetrate into the vitrified base plate for reoxidation of Ti ³⁺ to Ti ⁴⁺ during the practical time period. The comparative material V1 produced according to this comparative example is thus no longer suitable for use in EUV lithography. Thus, in contrast to the inventive method using steam conditioning treatment, oxygen treatment prior to vitrification does not result in the incorporation of oxygen to provide a storage effect.

As for the curve of the thermal expansion coefficient CTE as a function of temperature, the curve 1 of the embodiment 1 is not shown in Fig. 2 because it is changed to the comparison material V1, because the CTE curve is substantially defined by the titanium and fluorine content, and in the comparative example. 1 and the titanium and fluorine contents in Example 1 of the present invention are the same.

Comparative example 2

The TiO ₂ -SiO ₂ soot body was produced under the conditions explained in more detail in Example 1, however it contained only 7.4% by weight of TiO ₂ content. The TiO ₂ -SiO ₂ soot system was vitrified without a drying step and without a fluorine treatment. Conditioning treatment using water vapor or using oxygen is also omitted. After vitrification, the bottom plate contained an OH content of 250 wt. ppm. This relatively high OH content results in a 65% pre-formation and a relatively high transmission value of 85% even when formed and annealed with a ratio of small Ti ³⁺ /Ti ⁴⁺ . However, only such internal transmission values are not clear for eligibility in EUV lithography; rather, CTE curves and virtual temperatures must also be considered.

The CTE curve was determined after homogenization, shaping and annealing as described in Example 1. In Fig. 2, curve 2 shows that the CTE distribution of the comparative material V2 at this temperature is very steep. The gradient of CTE at a cross-zero temperature of 28 ° C in this case was 1.6 ppb / K ² . The virtual temperature (T _f ) was 930 ° C due to the absence of fluorine.

The substantial textures of the substrates manufactured according to Example 1 and the comparative materials V1 and V2 of Comparative Examples 1 and 2 according to the method of the present invention are summarized in the following table.

Claims (8)

A method of fabricating a titanium-doped glass substrate for EUV lithography having a high tannic acid content and having an internal transmission of at least 60% in the wavelength range of 400 nm to 700 nm at a sample thickness of 10 mm, titanium Having an oxidized form of Ti ³⁺ and Ti ⁴⁺ and having a given fluorine content, the method comprises the steps of: (a) producing a TiO ₂ -SiO ₂ soot body by flame hydrolysis of a ruthenium-containing and titanium-containing precursor material, (b) fluorinating the soot body to form a fluorine-doped TiO ₂ -SiO ₂ soot body, (c) treating the fluorine-doped TiO ₂ -SiO ₂ soot body in a vapor-containing atmosphere to form a conditioned Soot body, (d) vitrification of the conditioned soot body to form a high citric acid content, titanium doping having an average OH content in the range of 10 to 100 wt. ppm and an average fluorine content in the range of 2,500 to 10,000 wt. ppm The bottom plate of the glass. The method of claim 1, wherein the substrate is treated in a reducing atmosphere after vitrification, wherein the Ti ³⁺ /Ti ⁴⁺ ratio is increased while the internal transmission is reduced in the wavelength range of 400 nm to 700 nm. And wherein the bottom plate is subsequently annealed at a temperature between 600 ° C and 1000 ° C to restore the reduction in internal transmission. The method of claim 1 or 2, wherein the conditioning treatment using water vapor according to method step (c) is in the range of from 100 ° C to 1000 ° C, preferably from 500 ° C to 1000 ° C. The duration is 1 to 10 hours. The method of claim 1 or 2, wherein in the conditioning treatment using water vapor according to method step (c), the amount of water vapor in the inert gas is between 0.05% by volume and 50% by volume. Preferably, it is between 1 and 20% by volume. The method of claim 1 or 2, wherein the drying is carried out prior to fluorination according to method step (b), so that the average OH content is set to be less than 10 wt. ppm. The method of claim 1 or 2, wherein the fluorination according to method step (b) is carried out in a fluorine-containing atmosphere containing 2% by volume to 100% by volume of SiF ₄ . The method of claim 1 or 2, wherein the fluorination according to method step (b) is carried out at a temperature ranging from 700 ° C to not more than 1000 ° C. A bottom plate composed of titanium-doped glass of high tannic acid content for EUV lithography, having a transmission of at least 60% in a wavelength range of 400 nm to 700 nm at a sample thickness of 10 mm; The following method steps are made: (a) TiO ₂ -SiO ₂ soot body is produced by flame hydrolysis of barium-containing and titanium-containing precursors, and (b) the soot body is fluorinated to form fluorine-doped TiO ₂ -SiO ₂ soot. Body (c) treating the fluorine-doped TiO ₂ -SiO ₂ soot body in a vapor-containing atmosphere to form a conditioned soot body, and (d) vitrifying the conditioned soot body to form perrhenic acid the titanium content of doped glass plate; the average fluorine content in the range of 2,500 to 10,000wt.ppm; average OH content in the range of 10 to 100wt.ppm; and an average 6 wt% TiO ₂ content to 12 wt% The range in which the titanium is present in the oxidized form of Ti ³⁺ and Ti ⁴⁺ , and the ratio of Ti ³⁺ /Ti ⁴⁺ is adjusted to 2 × 10 ^-4 value.